The Sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) plays a dominant role in Ca2+ removal and is responsible for maintaining cardiac SR Ca2+ store. The activity of SERCA pump is regulated by two small molecular weight proteins, phospholamban (PLB) and sarcolipin (SLN). It is well documented that PLB is the key mediator of 2-adrenergic response in the ventricle. However, little is known about the role of SLN in cardiac SR calcium homeostasis. Recent studies from our laboratory have shown that SLN is predominantly expressed in the atria. Ectopic expression of SLN in the ventricular myocytes resulted in decreased calcium transients and myocyte contractility. Interestingly, the inhibitory effect of SLN was relieved upon isoproterenol treatment and stimulation at high frequency. Our studies therefore suggest that SLN is a novel regulator of cardiac SERCA pump. In addition, we found that SLN levels are significantly altered in diseased atria (heart failure and arrhythmia) of dogs and human, suggesting that an alteration in SLN /SERCA ratio could contribute to altered Ca2+ transport in failing myocardium. Based on these findings, we hypothesize that SLN is a key mediator of 2-adrenergic response in the atria and changes in its expression level may contribute to altered calcium homeostasis seen in atrial pathophysiology. In order to test these hypotheses, we have generated two transgenic mouse models 1) cardiac-specific over expression of SLN and, 2) SLN knockout (KO).
Aim I will test the hypothesis that SLN is the major regulator of SERCA pump and mediates the 2-adrenergic regulation of Ca2+ transport in atria using SLN transgenic, SLN KO and PLB KO mouse models.
Aim II will test the hypothesis that SLN action on SERCA pump is direct and its inhibitory function is regulated by phosphorylation and dephosphorylation status. The role of SLN phosphorylation and its interaction with SERCA pump will be assessed using site directed mutagenesis, and adenoviral gene transfer into cardiac myocytes. In addition, we will determine how SLN interaction with SERCA pump affect the kinetics of the SR calcium uptake.
Aim III will test the hypothesis that alterations in SLN to SERCA ratio will affect atrial function and predispose the atria to develop atrial pathology including atrial fibrillation upon increased load or stress. We will study the effect of pressure overload induced heart failure and increased pacing of the heart in SLN overexpressing and knockout mice. These studies will provide critical information on the role of SLN in atrial calcium handling and pave the way towards identifying novel therapeutic targets for treating atrial dysfunction including atrial fibrillation.
We recently identified a novel molecule namely sarcolipin. It is found predominantly in the atrial chamber of the heart and our studies indicate that it may regulate cardiac calcium transport during the beat to beat function of the heart. A major goal of this research proposal is to understand how Sarcolipin regulates Calcium transport and contractility of the atrial muscle. In addition another important goal of this study is to understand its role in atrial pathology. These studies will employ genetically engineered mouse models to understand if loss of Sarcolipin protein has an effect on cardiac function.
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